In the control of plant virus diseases, it is the general practice to make use of passive measures such as a pulling-out of infected plants or control of virus-transmitting insects such as aphides, rice insects, leafhoppers, nematodes and the like. Since, however, these measures are not to control the virus diseases positively, there has been long desired a development of chemicals for effectively controlling the virus diseases. A number of substances which show some activity against plant-viral diseases are known including, for example, antimetabolites such as 2-thiouracil, 5-fluorouracil, 8-azaguanine and the like, antibiotics such as blasticidin S, formycin B, aabomycin A and the like, polysaccharides or proteins produced by microorganisms, and polysaccharides, proteins and tannins derived from plants. Of these, the antimetabolites or the antibiotics are disadvantageous in that they produce chemical injury on plants and show no long-lasting efficacy. On the other hand, the polysaccharides or proteins or tannins produced by microorganisms or derived from plant have a disadvantage that they can not block viral diseases from vectors such as aphides.